<label>计时:</label> 
<input type="text" name="" id="timer"/> 
<button onclick="pause(this)" id="pause" state="on">暂停</button>
<button onclick="restart()">重新开始</button>

var ele_timer = document.getElementById("timer");
var n_sec = 0; //秒
var n_min = 0; //分
var n_hour = 0; //时

//60秒 === 1分
//60分 === 1小时
function timer() {
 return setInterval(function () {

 var str_sec = n_sec;
 var str_min = n_min;
 var str_hour = n_hour;
 if ( n_sec < 10) {
  str_sec = "0" + n_sec;
 }
 if ( n_min < 10 ) {
  str_min = "0" + n_min;
 }

 if ( n_hour < 10 ) {
  str_hour = "0" + n_hour;
 }

 var time = str_hour + ":" + str_min + ":" + str_sec;
 ele_timer.value = time;
 n_sec++;
 if (n_sec > 59){
  n_sec = 0;
  n_min++;
 }
 if (n_min > 59) {
  n_sec = 0;
  n_hour++;
 }


 }, 1000);
}

var n_timer = timer();

//暂停和继续
function pause(self) {
 var state = self.getAttribute("state");
 if (state === "on") {
  clearInterval(n_timer);
  self.textContent = "继续";
  self.setAttribute("state", "off");
 } else {
  n_timer = timer();
  self.textContent = "暂停";
  self.setAttribute("state", "on");
 }
}

function restart() {
 clearInterval(n_timer);
 n_sec = 0;
 n_min = 0;
 n_hour = 0;
 n_timer = timer();

 var ele_pause = document.getElementById("pause");
 ele_pause.textContent = "暂停";
 ele_pause.setAttribute("state", "on");
}